Recently, the popularity of mobile communication devices which utilize wireless transmission equipment, such as portable telephones, has become widespread. The wireless transmission equipment for such mobile communication devices is provided with antennas of various types for transmitting and receiving radio waves. Some commonly-used types of antennas include a top-helical type antenna consisting of a combination of a helical antenna and a whip antenna. The helical antenna functions when the whip antenna is retracted within an enclosure of the transmission equipment, and the whip antenna functions when the helical antenna is in an extended position, allowing ease of portability.
A top-helical type antenna of the prior art is illustrated in FIGS. 1 and 2.
FIG. 1 depicts a cross-sectional view of a prior art top-helical type antenna, wherein a helical antenna element 1 of an electrically conductive metal wire such as copper and a copper alloy is wound in coil form, a bobbin 2 is made of insulating polymeric resin and a feeding fixture 3 is made of electrically conductive metal. The helical antenna element 1 is wound on the bobbin 2 and is electrically connected at its lower end to the feeding fixture 3. A cover 4 of insulating resin encloses helical antenna 5.
Further, a conductor 1 made of electrically conductive metal is disposed within a resin-insulated tube 7 that forms a sleeve on an outer surface of the conductor 6. Conductor 6 is also electrically connected at a lower end to stopper 8, also made of electrically conductive metal, to comprise a whip antenna 9. Helical antenna 5 is joined at its lower end to the upper end of whip antenna 9 by insulator 10.
An antenna holder 11 made of electrically conductive material is provided with a through hole 11A at its center, and an outer periphery having threads 11B. The whip antenna 9 is inserted into the through hole 11A, and an outer surface of the stopper 8 contacts with an inner surface of the through hole 11A via a spring 12 fitted within the through hole 11A. This top-helical type antenna 14 is mounted to an enclosure 16 of wireless transmission equipment 15 by feeding nut 13 screwed into the threads 11B of the antenna holder 11.
Radio waves are transmitted and received by the whip antenna 9 that is electrically connected through contact stopper 8 with the antenna holder 11 fitted to the wireless transmission equipment 15, as the top-helical type antenna 14 is extended. When the whip antenna 9 is stored within the enclosure 16 of the wireless transmission equipment 15 by being pushed inside of the equipment 15, as shown in FIG. 2, the stopper 8 of the antenna 9 separates from the antenna holder 11, and the feeding fixture 3 of the helical antenna 5 makes contact with the antenna holder 11 for electrical connection, so that the radio waves are transmitted and received by the helical antenna 5.
However, prior art top-helical type antennas have a problem in that the impedance characteristic properly corresponding to the objective frequency band of radio waves is not easily obtainable due to winding diameters and winding pitches that are subject to dimensional dispersion during winding on the bobbin 2. This problem occurs because the helical antenna element 1 of the helical antenna 5 comprises a conductive metal wire wound in a coil form.